Field of the Invention and Related Art Statement
The present invention relates to an ultrasonic microscope apparatus, particularly to an ultrasonic microscope apparatus using cryogenic fluids, such as liquid nitrogen, liquid argon and liquid helium, as an ultrasonic propagating medium.
Heretofore, there has been utilized an ultrasonic microscope apparatus in which a specimen to be observed is scanned in two-dimensional directions by an ultrasonic beam and an acoustic image of the specimen is obtained by receiving the ultrasonic wave reflected from the specimen or transmitted through the specimen. In the ultrasonic microscope apparatus mentioned above, it has been necessary to improve the resolving power for the image of the specimen in order to obtain more accurate image data from the specimen.
The plane resolving power of ultrasonic microscope apparatus depends on the wavelength of the acoustic wave propagating through the ultrasonic propagating medium. There is a relation of c=f..lambda., among the velocity c, the frequency f and the wavelength .lambda. of an acoustic wave in the medium. Therefore, in order to improve the resolving power, i.e. in order to make the wavelength of the acoustic wave shorter, one can either to make the frequency of the acoustic wave higher or use an ultrasonic propagating medium having a lower velocity of sound. Since water is used as an ultrasonic propagating medium in the known ultrasonic microscope apparatus, the resolving power has been improved by former means, i.e. making the frequency of the acoustic wave higher. However, in practice, there is a limitation in making the frequency higher, because the amount of absorption of the acoustic wave propagating through the ultrasonic propagating medium is proportional to the square of the frequency f of acoustic wave. That is to say, in order to obtain a large enough S/N for the image of a specimen, it is necessary to receive reflected waves having an intensity higher than a predetermined level. Therefore, in making the frequency of acoustic wave higher, it is necessary to shorten the distance over which the acoustic wave propagates in the ultrasonic propagating medium, so that the intensity of reflected wave is not decreased by the absorption. This means that the so-called working distance of an acoustic lens used in the ultrasonic microscope apparatus should be small, in other words, the radius of curvature of the acoustic lens has to be small.
In the ultrasonic microscope apparatus being utilized today, a resolving power of 0.7.about.0.5 .mu.m is obtained under the frequency of 1.5 GHz.about.2.0 GHz. Such resolving power corresponds to the circumstance where a specimen is observed by a general optical microscope in which the radius of curvature of the objective is about 50 .mu.m.about.30 .mu.m and its working distance is about 30 .mu.m.about.10 .mu.m. In order to obtain the more qualified resolving power in the ultrasonic microscope apparatus, it is necessary to make the working distance of the acoustic lens smaller. However, this requirement could not be satisfied due to the difficulties in manufacturing the acoustic lens and in using the apparatus.
As described above, the resolving power of ultrasonic microscopes using water as the ultrasonic propagating medium has now become substantially identical with that of optical microscopes. However, a more improved resolving power, which is higher than that of the optical microscope, is required for ultrasonic microscopes when observing electronic devices, for example, very large scale integration elements or ceramic elements. Then, it is necessary to develop a ultrasonic microscope apparatus having a higher resolving power.
In order to increase the resolving power development of an ultrasonic microscope apparatus using cryogenic fluid, for example, liquid nitrogen, liquid argon or liquid helium, as an ultrasonic propagating medium, in which the acoustic velocity c and the absorption amount of the acoustic wave are smaller than those of water has been considered. Such an ultrasonic microscope apparatus using cryogenic fluid as the propagating medium is mentioned in the Journal of Acoustic Society of America, vol. 67 (1980) pp. 1629.about.1637. This known ultrasonic microscope apparatus comprises a heat insulated vessel, a stand for specimen arranged at the bottom of the heat insulated vessel, and an acoustic lens which is arranged above the stand, so that the specimen is scanned in two dimensions. Since this known ultrasonic microscope apparatus was developed only for the purpose of recognizing experimentally that its resolving power is much better than that of the apparatus using water, it has some drawbacks when putting it to practical use as described in the following.
The first drawback is that the field of view becomes small because the field of view is limited to about 40 .mu.m.times.30 .mu.m in order to obtain the desired resolving power. Hence it is difficult to adjust the position of the specimen to be observed into the field of view. For instance, in the case of testing an IC to find defects thereof by using the ultrasonic microscope apparatus, a field of view having the dimension of at least 1 mm.times.1 mm is necessary to adjust the position of IC for practical use. Therefore, the adjustment of the specimen's position is difficult in the known ultrasonic microscope apparatus.
The second drawback is that the operation for exchanging specimens is troublesome. When mounting a specimen after the observation for one specimen is concluded, it is necessary to take off the cap of the heat insulated vessel and remove the stand on which the acoustic lens, the supporting member therefor and the specimen are arranged, to the outside of the vessel. In this case, when the acoustic lens and the supporting member therefor are taken out of the cryogenic fluid to the outside, the moisture of outside air will freeze on the surfaces of these members in a moment and these members could not be used again. Also, when the cap of heat insulated vessel is removed, the moisture of the outside air enters into the vessel. Then the entered moisture might adhere onto the surface of the acoustic lens and specimen and thus forming a layer of water or ice thereon, so that it will be difficult to observe the specimen exactly. Further, when the next specimen is mounted into the heat insulated vessel, the specimen is cooled. Therefore, the moisture of the outside air that entered into the vessel will freeze on the surface of the specimen and thus it is impossible to observe exactly the specimen.
Moreover, in order to obtain an ultrasonic image having a high resolving power, it is necessary to adjust the ultrasonic beam exactly focused by the acoustic lens to the specimen. For this purpose, it is necessary to adjust the distance between the acoustic lens and the specimen from the outside under conditions where the heat insulated vessel including the acoustic lens and specimen therein is tightly closed.
And furthermore, cryogenic fluid such as liquid nitrogen, liquid argon, and liquid helium is boils under normal atmospheric pressure, and it is necessary to observe the specimen without the fluid boiling. If the cryogenic fluid is boiling when observing a specimen, the acoustic lens and the specimen are vibrated so that a clear ultrasonic image can not be obtained. In this case, boiling may be avoided if the heat insulated vessel containing the cryogenic fluid is closed up tight while observing and the vapor pressure of the space in contact with the surface of the cryogenic fluid is made higher than the normal atmospheric pressure, because the boiling point of a cryogenic fluid depends on vapor pressure.
However, if observation is continued with the vessel closed up tight, the vapor pressure of the space in contact with the surface of the cryogenic fluid will continue to increase, because the cryogenic fluid continues to evaporate according to the inflow of heat from the outside. The supporting member for the sample rod of the apparatus will be deviated thereby, and thus the position of the sample rod will be also deviated, so that adjustment of the focus of the acoustic lens to the specimen cannot be achieved. And, there is a danger that all sorts of members of the apparatus might explode by increasing the vapor pressure inside the vessel.